Generally, rotary transformers have been widely used in rotary magnetic head type video tape recorders to achieve reception and transmission of recorded and reproduced video signals by a rotary magnetic head provided with a rotary drum. Conventionally, the rotary transformer which performs signal transmission by using electromagnetic coupling between coils has a narrow band in which signals can be transmitted due to influences of the inductances of coils and stray capacitance. The frequency limit was about 60 MHz.
Recently, rotary couplers have been demanded in which the frequency band of signals to be processed has been widened in association with the development of digital VTR, data recorder, high definition TV, etc.
The present applicant has previously proposed a microstrip line type rotary coupler which is capable of transmitting signals over a wide band by forming microstrip lines serving as a directional coupler on opposing faces of rotary and stationary portions as is disclosed by, for example, Japanese Examined Patent Publication No. 64-9763.
The rotary coupler, the basic structure of which is disclosed in the Japanese Examined Patent Publication No. 64-9763, includes a rotary portion rotatable around the center of a first circumference, having a first microstrip line which is formed by a first microstrip disposed along the first circumference and an electrically conductive layer disposed in parallel with the first microstrip via a dielectric material layer and is terminated by a non-reflecting terminating resistor at one end thereof, a stationary portion having a second microstrip line which is formed by a second microstrip disposed along a second circumference around said center of rotation so that the second microstrip faces and is close to said first microstrip and formed by an electrically conductive layer disposed in parallel with said second microstrip via a dielectric material layer, said second microstrip line being terminated by a non-reflecting resistor at one end thereof, an input circuit connected with the other end of one of said first and second microstrips for inputting radio frequency signals thereto and an output circuit connected with the other end of the other one of said first and second microstrip lines for picking up the radio frequency signals inputted by said input circuit. The length of an electromagnetically coupled portion, by which said first microstrip is electromagnetically coupled with said second microstrip, is a multiple of an odd number of substantially 1/4 of the electrical wave length of the radio frequency signal which propagates through the electromagnetically coupled portion, whereby reception and transmission of the radio frequency signals is performed between said rotary and stationary portions by the electromagnetic coupling between the first and second microstrips.
If a structure in which an end of the microstrip line is terminated by an non-reflecting resistor is adopted in such a microstrip line type rotary coupler, the microstrip line would not become a closed loop. Accordingly, a continuous signal transmission cannot be performed, resulting in changes in amplitude due to rotation of the rotary portion.
Therefore, the present applicant has proposed as a rotary coupler in which an improvement for reducing the changes in amplitude due to rotation of the rotary portion is achieved, a rotary coupler in which one of microstrip lines is formed along substantially one lap of the circumference having a circumferential length equivalent to a multiple of an integer of the central frequency of a transmission frequency band as is disclosed in the Japanese Examined Patent Publication No. 61-105903, a rotary coupler in which at least either one of microstrip lines on the side of a rotary portion or a stationary portion is divided into a plurality of sections, formed along the substantial entire circumference of the rotary coupler, as is disclosed in Japanese Unexamined Patent Publication No. 63-220401, and a rotary coupler in which one of microstrip lines is formed along the circumference of the rotary coupler having a circumferential length equivalent to a multiple of an integer of the central frequency of a transmission frequency band in electrical length and is bent at both ends, and is extended, and is provided with a connecting end and a non-reflecting terminating resistor on positions external to the circumference, as is disclosed in Japanese Unexamined Patent Publication No. 2-30202.
However, the various rotary couplers which are improved for reducing the changes in amplitude of transmission characteristics due to rotation of the above-mentioned rotary portion have problems in that a high precision is required for a pattern of the microstrips and that the pattern of the microstrips or the structure of external circuit, etc becomes complicated.
In the previously proposed various rotary couplers, one of the microstrip lines is formed along substantially one lap of the circumference of the rotary coupler having a circumferential length equivalent to a multiple of an integer of the electrical wave length of the transmitting radio frequency signal. It has been found from experiments seeking improvement in reducing the changes in amplitude in transmission characteristics due to rotation of the rotary portion that the changes in amplitude will become very low by forming one of the microstrips along substantially one lap of the circumference having a circumferential length equivalent to substantially 1/2 of the electrical wave length of the transmission radio frequency signal.